This invention pertains to microlithography (transfer of a pattern, defined on a reticle or mask, onto a sensitive substrate). Microlithography is a key technology used in the fabrication of semiconductor integrated circuits, displays, micromachines, and the like. More specifically, the invention pertains to devices (termed xe2x80x9cwafer chucksxe2x80x9d), to which the substrate (xe2x80x9cwaferxe2x80x9d) is mounted, that hold and move the substrate during microlithographic exposure. Even more specifically, the invention pertains to wafer chucks and related substrate-holding devices operable to correct instances of insufficient adhesion of the substrate to the substrate-holding device.
During microlithographic exposure of a sensitive substrate (xe2x80x9cwaferxe2x80x9d) the wafer typically is mounted to and held by a xe2x80x9cwafer chuck.xe2x80x9d Microlithography performed using a charged particle beam must be performed in a subatmospheric pressure (xe2x80x9cvacuumxe2x80x9d) environment; hence, the wafer chuck must be capable of holding the wafer in such an environment. Most conventional wafer chucks intended for use in a vacuum environment are configured to hold the wafer using electrostatic force. The surface of the wafer chuck to which the wafer (i.e., the downstream-facing surface of the wafer) is mounted is termed the xe2x80x9cadhesion surfacexe2x80x9d of the wafer chuck.
During exposure of a wafer using a charged particle beam, the exposure beam is incident with high energy on the xe2x80x9csensitivexe2x80x9d surface (upstream-facing resist-coated surface) of the wafer. Consequently, the wafer tends to experience heating, which can cause undesired thermal expansion of the wafer. Thermal expansion of the wafer can degrade the accuracy with which a pattern is transferred to the sensitive surface. Under extreme circumstances of wafer heating, the wafer can detach from or shift position on the adhesion surface.
A conventional method of reducing wafer heating is to configure the adhesion surface with grooves or channels that open onto the adhesion surface and the downstream-facing surface of the wafer. A heat-transfer gas such as helium is conducted through the channels, whenever the wafer is mounted to the adhesion surface, to dissipate heat from the wafer and thus reduce thermal expansion of the wafer.
To ensure that the wafer remains attached to the adhesion surface as the heat-transfer gas is passed through the channels, the pressure of the heat-transfer gas passing through the channels is regulated. In other words, the pressure of the gas must be less than a pressure, opposing the electrostatic force, sufficient to detach the wafer from the adhesion surface. Meanwhile, parameters that determine the quantity of heat transferred from the wafer to the wafer chuck by the heat-transfer gas include the thermal conductance of the gas, the gas pressure, and the length of the channel(s) through which the gas passes. For example, if the gas pressure is sufficiently low that the mean free path of the gas molecules is longer than a transverse dimension of the channel, then the thermal conductivity of the heat-transfer gas increases nearly proportionally to the gas pressure. On the other hand, if the mean free path is shorter than a transverse dimension of the channel, then the thermal conductivity is not proportional to the gas pressure.
Because the wafer chuck normally is located in a subatmospheric pressure environment, as the pressure of the heat-transfer gas in the channel increases, adhesion of the wafer to the adhesion surface of the wafer chuck weakens. In the worst case, the wafer actually detaches from the wafer chuck. Hence, it is important to maintain the pressure of the heat-transfer gas in the channel below a threshold that otherwise would result in detachment of the wafer from the adhesion surface.
The mean free path of molecules of the heat-transfer gas is obtained from an estimate of the pressure of the heat-transfer gas. In view of this, it is desirable to configure the channel (in the adhesion surface and located between the wafer chuck and the downstream-facing surface of the wafer) to have transverse dimensions that are equal or nearly equal to the mean free path.
With a conventional electrostatic wafer chuck, after the chuck is charged electrostatically, the wafer is assumed to be adequately adhered to the adhesion surface and the flow of heat-transfer gas through the channel begins. But, if the wafer in fact is not adhered adequately to the wafer chuck, even if the pressure of the heat-transfer gas is regulated xe2x80x9cnormally,xe2x80x9d a substantial risk exists that the wafer will xe2x80x9cfloatxe2x80x9d and laterally shift position on the adhesion surface. Other adverse consequences are also possible, such as the wafer actually falling off the wafer chuck. If any of these adverse events occurs, then the vacuum inside the chamber enclosing the wafer chuck must be broken and the wafer removed by hand. Afterward, the process of re-establishing the vacuum in the chamber and re-mounting the wafer to the wafer chuck must be performed, which results in lengthy equipment down-time.
Other possible adverse conditions are the presence of particulate debris between the downstream-facing surface of the wafer and the adhesion surface as the wafer is resting on the adhesion surface, and poor planarity or flatness of the wafer itself. As noted above, the flow of heat-transfer gas into the channel is regulated to maintain a particular target pressure of the gas in the channel under normal conditions. But, either of the adverse conditions noted above essentially opens the channel and allows excess leakage of heat-transfer gas from the channel into the vacuum chamber.
In view of the disadvantages of conventional wafer chucks as summarized above, an object of the invention is to provide substrate-holding devices (generally termed herein xe2x80x9cwafer chucksxe2x80x9d) configured to prevent insufficient adhesion of the wafer to the wafer chuck. Another object is to provide microlithography apparatus including such improved wafer chucks.
To such ends and according to a first aspect of the invention, substrate-holding devices are provided. An embodiment of such a device includes a wafer-chuck body that defines an adhesion surface and comprises an electrostatic electrode. The adhesion surface is configured to contact a downstream-facing surface of a substrate whenever the substrate is being held by the substrate-holding device by an electrostatic force generated by the electrode. The adhesion surface defines a channel that is configured, whenever the substrate is adhered to the adhesion surface by the electrostatic force, to provide a conduit for a heat-transfer gas. Hence, whenever the heat-transfer gas is flowing through the conduit, the gas contacts and removes heat from the downstream-facing surface of the substrate. The device also comprises a gas-supply system and a substrate-adhesion-confirmation device. The gas-supply system is connected to the channel and configured to supply a flow of the heat-transfer gas to the channel. The substrate-adhesion-confirmation device is situated and configured to detect whether the substrate is adhered to the adhesion surface. The device also includes a controller connected to the substrate-adhesion-confirmation device and to the gas-supply system. The controller is configured to cause the gas-supply system to supply the flow of the heat-transfer gas to the channel after the substrate-adhesion-confirmation device has confirmed adhesion of the substrate to the adhesion surface.
By way of example, the substrate-adhesion-confirmation device can comprise a height gauge situated and configured to measure an elevation of the substrate. Alternatively, the substrate-adhesion-confirmation device can comprise multiple grounding pins each situated and configured to contact the substrate electrically in a manner whereby a contact resistance of the electrical contact varies with contact pressure exerted by the respective grounding pin on the substrate. In this latter configuration, a power supply is provided that is connected via an electrical circuit to the grounding pins. The device, utilizing the power supply, can be configured to provide the requisite confirmation by any of the following schemes: (1) grounding of the substrate via the grounding pins and measuring changes in contact resistance of the pins with changes in contact pressure of the substrate against the grounding pins; (2) impressing a voltage between any pair of grounding pins and measuring voltage changes with changes in contact pressure of the substrate against the grounding pins; and (3) flowing an electrical current between any two of the pins and measuring the current. In general, if the contact pressure of a pin against the downstream-facing surface changes, then the contact resistance between the pin and the substrate changes, leading to a corresponding change in, e.g., electrical current between any two pins. Also, since the substrate is grounded by the pins, charging of the substrate during irradiation by a charged particle beam can be prevented.
If a grounding pin is not actually contacting the downstream-facing surface of the substrate, then electrical current will not flow through the circuit including the pin. Thus, it can be checked readily whether a pin is contacting the downstream-facing surface firmly.
Hence, according to the invention, it is not simply presumed that the substrate is adhered properly to the holding device after the electrode is energized. Rather, a confirmation is made that the substrate is adhered properly to the holding device. If no confirmation is made, then heat-transfer gas is not delivered to the channel. Consequently, problems associated with poor wafer chucking are avoided.
The device also can include a substrate-alignment device situated and configured to maintain a predetermined alignment position of the substrate relative to the adhesion surface under conditions in which the substrate is not actually adhered to the adhesion surface. The substrate-alignment device can comprise multiple alignment pins situated around the adhesion surface and configured to contact a respective edge of the substrate if the substrate moves laterally relative to the adhesion surface.
Desirably, the alignment pins are made of a non-magnetic metal, and are attached to the wafer chuck around a perimeter of the adhesion surface. By making the alignment pins of a non-magnetic metal, a charged particle beam impinging on the substrate being held by the substrate-holding device is unaffected by extraneous magnetic fields that otherwise would be formed.
According to another aspect of the invention, methods are provided for holding a substrate to allow performing a process on a process surface of the substrate. In an embodiment of such a method, a wafer chuck is provided that comprises a chuck body, an electrostatic electrode, and an adhesion surface on the chuck body. The adhesion surface defines a channel configured, whenever the substrate is adhered to the adhesion surface, to provide a conduit for a heat-transfer gas that, when flowing through the conduit, contacts and removes heat from the downstream-facing surface of the substrate. The substrate is placed on the adhesion surface, and the electrode is energized to cause the electrode to generate an electrostatic force intended to attract the substrate toward the adhesion surface. A confirmation is made that the substrate is adhered to the adhesion surface. After making the confirmation, a flow of the heat-transfer gas through the channel is commenced.
In another method embodiment, a wafer chuck is provided that includes the components summarized above as well as a substrate-alignment device. The substrate-alignment device is situated relative to the chuck body and is configured to contact a respective edge of the substrate if the substrate moves laterally relative to the adhesion surface. The substrate is placed on the adhesion surface and the electrode is energized. A flow of the heat-transfer gas through the channel is commenced. Using the substrate-alignment device, respective edges of the substrate are contacted as required to prevent the substrate from laterally sliding off the adhesion surface as a result of the heat-transfer gas flowing through the channel.
The foregoing methods can be utilized in conjunction with a method for manufacturing a microelectronic device on a substrate. For example, the manufacturing method can be a microlithographic method.
Another embodiment of a substrate-holding device comprises a wafer chamber defining an interior space. A pump is connected to the wafer chamber and configured to evacuate the interior space to a predetermined vacuum level. A wafer-chuck is situated within the interior space. The wafer chuck defines an adhesion surface and comprises an electrostatic electrode. The adhesion surface is configured to contact a downstream-facing surface of a substrate whenever the substrate is being held by the substrate-holding device (by an electrostatic force generated by the electrode). The adhesion surface defines a channel configured, whenever the substrate is adhered to the adhesion surface by the electrostatic force, to provide a conduit for a heat-transfer gas that, when flowing through the conduit, contacts and removes heat from the downstream-facing surface of the substrate. The device includes a gas-inlet conduit connected to the channel and to a supply of the heat-transfer gas, wherein the gas-inlet conduit is configured to conduct the heat-transfer gas into the channel. The device includes a gas-evacuation conduit connected to the channel and to an evacuation pump, wherein the gas-evacuation conduit is configured to evacuate, as urged by the evacuation pump, the heat-transfer gas from the channel. The device includes a seal situated relative to the substrate, as held on the adhesion surface, and the channel.
The seal is configured at least to limit an amount of heat-transfer gas flowing from the channel to the interior space whenever the substrate is on the adhesion surface. The device includes a first pressure detector situated and configured to measure a vacuum level in the interior space, and a second pressure detector situated and configured to measure a pressure within the gas-inlet conduit. The device also includes a controller connected to the first and second pressure detectors, to the supply of heat-transfer gas, and to the evacuation pump. The controller is configured to regulate a flow rate of the heat-transfer gas though the gas-inlet conduit in response to a pressure, detected by the second pressure detector, within the gas-inlet conduit, and to regulate a flow rate of gas through the gas-evacuation conduit in response to a vacuum level in the interior space as detected by the first pressure detector.
The device summarized above also can include a third pressure detector connected to the gas-evacuation conduit and to the controller. With such a configuration, the controller is further configured to regulate the flow rate of heat-transfer gas through the gas-inlet conduit in response to a pressure, detected by the third pressure detector, in the gas-evacuation conduit.
According to another aspect of the invention, charged-particle-beam (CPB) microlithography apparatus are provided that include a CPB-optical system situated and configured to direct a charged particle beam to form an image on a surface of a substrate. The apparatus also include a vacuum chamber defining an interior space, a vacuum pump connected to the wafer chamber and configured to evacuate the interior space to a predetermined vacuum level, and a wafer chuck. The wafer chuck is situated within the interior space. The wafer chuck defines an adhesion surface and includes an electrostatic electrode as summarized above. The wafer chuck also includes a gas-inlet conduit, a gas-evacuation conduit, a seal, a first pressure detector, a second pressure detector, and a controller, all as summarized above.
In an embodiment, according to the invention, of a method for manufacturing a microelectronic device on a substrate, a method is included for holding the substrate to allow performing a process on a process surface of the substrate. In such a method, a vacuum environment is provided in which to hold the substrate while performing the process on the substrate. A predetermined vacuum level is established in the vacuum environment. In the vacuum environment a wafer chuck is provided that comprises a chuck body, an electrostatic electrode, an adhesion surface, and a seal as summarized above. The substrate is placed on the adhesion surface. The electrode is energized to cause the electrode to generate an electrostatic force intended to attract the substrate toward the adhesion surface. Heat-transfer gas is conducted at a first flow rate through a gas-inlet conduit to the channel. Heat-transfer gas is evacuated at a second flow rate from the channel through a gas-evacuation conduit. The vacuum level in the vacuum environment is detected to obtain respective data, and a gas pressure within the gas-inlet conduit is detected to obtain respective data. Responsive to this data, the first flow rate and the second flow rate are regulated.
If adhesion of the substrate to the adhesion surface is poor, then the leakage of heat-transfer gas from the channel into the vacuum environment is increased compared to when adhesion is good and proper. Hence, whenever adhesion is poor, less vacuum is detected in the vacuum environment. The second flow rate is controlled (in this instance increased) to return to high vacuum in the vacuum environment, desirably bringing the vacuum level to a target value. Thus, leakage of heat-transfer gas from the channel is compensated for. Meanwhile, the first flow rate also is controlled to bring the detected pressure in the gas-inlet conduit at least closer to (desirably at) the target value.
It is desirable to consider the pressure in the gas-evacuation conduit in the regulation of the flow rate in the gas-inlet conduit, so as to improve estimates of the pressure inside the channel.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.